Academic Year Electromagnetism

High School Mathematics-Physics SMILE Meeting
1997-2006 Academic Years
Electromagnets and Motors

11 November 1997: Lee Slick [Morgan Park HS]
Motor (originally Rudy Keil's brainchild)
Making a simple DC motor with a 1.55V D cell and a coil with wire that is enameled, which must be sanded for proper contact.

09 December 1997: Jane Shields [Chicago SDA Academy]
She did a lesson on magnetism using the electric motor as a device to deflect compasses from pointing "true North". It was not easy to measure secondary currents even using a micro-ammeter, although the primary current in the motor did cause a compass needle to deflect.

She showed a galvanometer she made using a compass by wrapping bell wire around it, and a generator using a magnet and 20 turns of bell wire.

16 March 1999: Bill Blunk [Joliet Central HS]
He brought in a toy sold at Science stores - Levitron. See the websites http://www.levitron.com/ and http://www.physics.ucla.edu/marty/levitron/.
It consists of a magnet top spun over a magnet that levitate the top as long as it spins. Also showed why a device to spin the top is available. It was hard to get to top spinning so that it could be maneuvered to be over the base magnet. After it is positioned by what looked like a Petri dish, it would rotate and be held up by the magnetic field. For a discussion of the physics of Levitron, see http://www.levitron.com/physics.html. 20 April 1999: Bill brought in a sheet that reveals the magnetic fields and showed with a magnetic field sheet display that the Levitron base has a magnetic field that basically has a hole in the center.

27 February 2001 Earnest Garrison (Jones Academic Magnet HS, Physics)
addressed the problem of teaching Electricity and Magnetism to the Nintendo Generation. He first showed us a coil that produced a high voltage spark when touched to metal objects. He recommended using a Basic Electronic Component Kit produced by Elenco Electronics Inc http://www.elenco.com [along with a manual of experiments]:

Kit Model PK-101
Omnitron Electronics
600 S. Military Trail
Deerfield Beach Fl 33442
1 - 800 - 379-6664

The kit, which transforms any standard breadboard into an Electronics Learning Center, contains the following items:

transistor, diode, led,
capacitors, resistors, potentiometer,
wires, circuit board.

Earnest gave us handouts on two experiments:

#1: The Light Bulb
#2: The Brightness control

He used a digital multi-meter, and a crank generator with the kit as accessories. He also described hooking the battery to a crank generator, which causes it to run backwards. He felt it was important to bring current "chip technology" into the classroom. He described a conducting material as being like Cheerios™ in milk, and an insulator as being like dry Cheerios™ . He described eddy currents set up in a copper tube, that caused it to fall slowly through a region of strong magnetic field, to illustrate that electric currents produce magnetic fields, and vice versa. Good, Earnest!

14 March 2001 Arlyn VanEk (Illiana Christian HS, Physics)
illustrated the behavior of electric motors and generators, using the camera probe [http://www.allelectronics.com/] with the large-screen TV. First, he reminded us that the directions of Current and Magnetic Field are related by the right-hand rule. Using your right-hand:

Point your thumb in the direction of the conventional current, I [labeled by v in the diagram!]
Curl your fingers into a half-circle around the wire. They point in the direction of the magnetic field, B.

[Source: http://purcell.phy.nau.edu/SeatExpts/resource/rhr/righthandrule.htm]. He showed how to give a large-scale visualization of the convention by putting long paper cylinders on the fingers of his right hand. He then used visuals to illustrate how a current-carrying loop of wire is made to rotate in a magnetic field, and noted that if the current does not vary with time [Pure DC], the wire will not continue to rotate, but will come to rest at an equilibrium position. The changing current necessary to make an electric motor can be obtained with a split ring [commutator], so that the current changes direction every half-rotation. The current will be of fixed magnitude, but its direction will change every half-cycle.

Next he showed how to make an electric generator to convert mechanical energy into electrical current. The device is a rotating split-ring coil in an external magnetic field, so that the direction of flow of current reverses at every half-turn. The output voltage of 3-5 volts was monitored on a standard digital oscilloscope, available in the Radio Shack Catalog [ http://www.radioshack.com/], such as this model:

100MHz Cursor Readout Dual-Channel Oscilloscope
$1,199.99 Reg. Price; Brand: Instek
Cat #: 910-5360 Model: GOS-6103

As seen from the trace, the output voltage is about 5 Volts with full wave rectified AC structure; that is, I(t) = I_o |sin w t|. In addition, the oscilloscope showed spiked voltage pulses that correspond to the openings in the split ring commutator. These pulses are similar to high frequency ignition interference that may be heard on AM channels on the car radio, which correspond to opening and closing of distributor points.

04 December 2001: Porter Johnson (IIT Physics) Generator or Alternator?
Porter passed out an article by Bob Weber [sobriquet Motormouth] that appeared in the Chicago Tribune [http://www.chicagotribune.com/] on 16 July 2001, entitled By alternate name, device still generates electricity. The article began with the following question::

Q: Years ago they were called generators, and now they are called alternators. What is the difference?

In answering the question, Bob / Motormouth had made the following points:

An old generator involves moving a coil of wire through a stationary magnetic field.
An alternator involves moving a magnetic field through a stationary coil of wire.
Both the old generators and the newer alternators produce alternating current, which is converted to direct current. In an old generator the conversion to DC is handled mechanically with brushes and commutators, whereas in an alternator it is handled electrically with diodes.
The Society of Automotive Engineers [SAE] recently agreed to call the automotive device that makes electricity a "generator". That makes sense, because it "generates", rather than "alternates". Hence, the answer to the question is that although the alternator is more efficient than the old generator---especially at lower speeds---both are now called generators.

22 October 2002: Bill Shanks [Joliet West Physics, retired] More Power 2 U!
Bill noticed that the Shop.Vac® wet-dry vacuums were very powerful indeed, in that the manufacturer apparently advertised product #971-01-00 as having a peak power of 1.5 to 3 Horsepower, while requiring an electric current of only I = 6 Ampères at V = 120 Volts. Indeed, such an assertion appears on the Shop Vac web page, http://www.shopvac.com. Bill showed by direct calculation that 1 Horsepower = 550 ft lb/sec = [550 ft lb/sec] ´ [0.304 m/1 ft] ´[ 9.8 Nt /2.205 lb] = 746 Watts. The input electrical power P = V ´ I = [120 Volts] ´ 60 Amps = [720 Watts], so that this vacuum system is a very remarkable one, indeed, since it can produce at least [1.5 Horsepower] ´ [746 Watts/Horsepower] = 1119 Watts.. Bill also mentioned that a 5 Horsepower Shop Vac required only 8 Amp at 120 Volts, an even more remarkable device! [Porter Johnson pointed out that the European standard horsepower is only 735 Watts, which still did not explain the discrepancy!] It appears that you have solved the energy crisis for once and for all, Bill!

11 March 2003: Ann Brandon [Joliet West HS, Physics] Producing Current in a Coil of Wire
Ann hooked a coil of wire in series with a fairly sensitive galvanometer, and then passed a cow magnet back and forth through the center of the coil. The galvanometer needle moved in response, indicating that an electric current was passing through the wire as the cow magnet moved. This is a direct illustration of Faraday's Law, in which the change of magnetic flux passing through a current loop is equal to the induced Voltage. Ann also described dropping a cow magnet through a hollow, vertically held Aluminum tube 5 meters in length. Normally the magnet would drop that distance in about 1 second in free fall, but it took about 11 seconds for the magnet to travel down the tube. This is because the falling magnet induces a current in the tube. The current -- in turn -- produces a magnetic field of its own, which opposes the field of the falling magnet, thus slowing its descent. [Lenz's Law: http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/MagneticField/LenzLaw.html]. Cow magnets are wonderful, although they don't look very appetizing!

Most interesting! We can always depend on Ann!

24 February 2004: Larry Alofs [Kenwood HS, Physics] "Armstrong" Flashlight
Larry held up a transparent, plastic flashlight, called the "Eternal Flashlight". When he shook it back-and-forth along its principal axis, it would generate enough energy to light up. We could see a solenoid coil near its center, and a magnet that shuttled back-and-forth along a tunnel passing through the solenoid. So the changing magnetic flux within the coil would generate an EMF producing a current which charged a capacitor storing energy to light the bulb. Visit the Heartland America website http://www.HeartlandAmerica.com, search for product number 95784 --- about $15. Larry then held up an LED flashlight, purchased for about $10 on the Harbor Freight website: http://www.harborfreight.com. The flashlight that he showed us had 3 AA batteries and 2 LEDs, The current drawn for this LED flashlight was very low, and it would operate continuously for essentially the shelf life of the batteries!

You really brightened up our day ... and our nights! Thanks Larry!

26 April 2005: Babatunde Taiwo [Dunbar HS] Simple Electric Motors
Babatunde showed an electric motor that he had built according to the instructions obtained from Pasco Scientific Teacher Resource Guide [Investigation Seven: Making a Model Motor from Scratch, pp T-285,286], using a D-cell battery, about 1.5 meters of magnetic wire, a battery holder, masking tape, a strong magnet, and a two paper clips. The following explanation is excerpted from that source:

"Stored chemical in the cell established an electric pressure difference which pushed charge through the coil -- but only if the coil is oriented so that scraped ends are in contact with the clips. (If the insulated half of one end is in contact with the clips, no charge can flow.) Using the Right Hand Rule for Motors (7.11), once can predict the direction of the force on the wire on the two sides of the coil, and therefore which way the coil will rotate."

His students were able to construct a simple electric motor, complete with field magnet, rotor, and commutator, in one class period by following these instructions. For details on a very similar procedure, see the lesson given by Lee Slick in SMILE on 11 November 1997 [ph1111.htm], and those given by Ann Brandon in SMILE on 23 March 2004 [mp032304.html].

Earl Zwicker informed us that descriptions of both of these experiments have appeared in The Physics Teacher TPT:

Spinning Loop by Rudy Keil TPT: Vol 23, p 308 (May 1985).
Spinning Screw/battery/magnet/wire by Chris Chiaverina TPT: Vol 40, p 553 (Dec 2004).